Heretofore, it has been customary to fill a cavity formed by an upper die and a lower die that are joined to each other, with a forging material, and applying a pressing force to the forging material through a punch, for thereby producing an outer race member (outer cup) of a constant-velocity joint for driving automotive wheels, for example.
The outer race member comprises a tubular cup and a shank integrally formed with the cup. The cup has three axially extending track grooves defined in an inner circumferential surface thereof, and rollers can roll in and along the track grooves.
With respect to the method of manufacturing an outer race member for constant-velocity joints of the type described above, Japanese Laid-Open Patent Publication No. 57-206537, for example, discloses a technical concept for solving the problem of a larger axial elongation of larger-diameter portions and a smaller axial elongation of smaller-diameter portions when a cup-shaped outer member blank having a wall thickness, which is substantially uniform dimension with that of the product dimension, is ironed. According to the disclosed technical concept, the dimensions of the cup-shaped outer member blank are set according to the constant volume law to keep axial elongations substantially constant.
Japanese Laid-Open Patent Publication No. 61-3618 discloses a technical concept for forming a cup-shaped rough product with a shank according to a forging process, the rough product having an inner surface shape which is substantially the same as a finished shape, then setting an inner die on the inner surface of the rough product, and ironing the entire circumference of an outer surface toward the inner surface at uniform ironing ratios in a uniform thicker portion, a uniform thinner portion, and a portion extending from the thicker portion to the thinner portion, thereby finishing a plurality of grooves in the inner surface with high accuracy.
According to the technical concept disclosed in Japanese Laid-Open Patent Publication No. 57-206537, however, since ironing ratios of the bottoms of track grooves, the track grooves, and the inner surface are largely different from each other, the accuracy of the groove surfaces of the track grooves is lower than if the bottoms of track grooves, the track grooves, and the inner surface are ironed with a uniform ironing ratio.
According to the technical concept disclosed in Japanese Laid-Open Patent Publication No. 61-3618, it is premised that the cup-shaped rough product with the shaft which has the inner surface shape which is substantially the same as the finished shape is used as a forging blank (workpiece). If the manufacturing process disclosed in Japanese Laid-Open Patent Publication No. 61-3618 is applied to a workpiece in the form of a cup-shaped intermediate blank that is produced by extruding a billet backwards and has a wall thickness difference between larger-diameter portions and smaller-diameter portions, then the thinner portion is axially extended greater than the thicker portion due to different ironing ratios. Even if the intermediate blank whose axial end face is irregular is ironed at a circumferentially uniform ironing ratio, the end face produced after the blank is ironed still remains irregular, and the amount of finishing work on the axial end face increases.